In this Ted talk deaf percussionist Evelyn Glennie discusses listening. Glennie demonstrates that there is a lot more to listening than just hearing the sound through our eardrums.
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Accent the second note of each double in an open roll to develop an even roll:
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>
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R
R
L
L
R
R
L
L
I don't remember where I first learned about this exercise of accent the second note of each double stroke but I've never played doubles any other way since. It's a "feel" thing. You have to feel the up and down motion of the sticks like Chapin is talking about in this video rather than the "digging in" that he's talking about.